Telechelic polymers, possessing reactive functional group(s) situated at the polymer chain end(s), are an importance class of polymeric materials. They find applications as prepolymers for inclusion into final products with well-specified properties by the reaction of their functional groups. One early example of the commercial use of such telechelic polymers is in the formation of polyurethanes, which can be perpared by coupling reaction between hydroxyl-terminated prepolymers with diisocyanates. This example introduced new perspectives on making materials with a wide array of physical properties by controlling the molecular architecture of the polymers.
Originally a telechelic polymer was considered a polymer containing two reactive end-groups, one group at each end. As the use for such materials has grown, so too has the definition of materials classified as telechelic polymers. The current broad definition of telechelic polymers include all polymers that contain one or more reactive end-group(s), which can undergo chemical reactivity with itself or another functional group in another molecule. The polymer that possesses only one reactive end-group is now referred to as a “monotelechelic” and the original telechelic having two opposing reactive end-groups is commonly called a “ditelechelic”. Those telechelic polymers having more than two reactive end-groups are designated as tritelechelic, tetratelechelic, or polytelechelic. By far, the most important commercial telechelic polymers are monotelechelic and ditelechelic polymers. These materials serve as prepolymers for preparing well-defined graft copolymers and multi-block copolymers, respectively. Telechelic polymers with higher functionality (greater than 2) usually result in materials having a polymer network structure. An important consideration in the use of telechelic polymers is their average functionality, i.e., the average functionality of a monotelechelic polymer should be 1.0 and that of a ditelechelic polymer should be 2.0. Typically end-group linking reactions are highly sensitive to accurate end group stoichiometry. The quality of graft and multi-block copolymers employing telechelics depend upon the preciseness of their functionality, 1.0 and 2.0, respectively.
Most monotelechelic vinyl polymers are prepared by terminating living polymers with suitable reagents. Practically all vinyl polymerization mechanisms, including anionic, cationic, free radical, metathesis, and Ziegler-Natta, have shown living polymerizations with stable propagating active sites that can be converted to a desired functional group at the chain end.
In many respects, free radical polymerization is the most important commercial process of producing vinyl polymers. However, the facile radical coupling and disproportional reactions offer little or no control over the propagating sites. Early attempts to realize a living free radical polymerization involved the concept of reversible termination of the growing polymer chains by iniferters, such as N,N-diethyldithiocarbamate derivatives (Otsu, et al., Macromol chem., Rapid Commun., 3, 133, 1982, Eur. Polym. J., 25, 643, 1989). The first robust living radical polymerization was observed in reaction involving a stable nitroxyl radical, such as 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), that does not react with monomers but forms a reversible end-capped propagating chain end (see, Georges, et al., U.S. Pat. Nos. 5,322,912 and 5,401,804). The formed covalent bonds reduce the overall concentration of free radical chain ends, which leads to a lower occurrence of unwanted coupling and disproportionation termination reactions. For an effective polymerization, the reaction has to be carried out at an elevated temperature (>100° C.). Relatively high energy is needed in the cleavage of the covalence bond, which maintains a sufficient concentration of propagating radicals for monomer insertion. This living radical polymerization, however, appears effective only with styrenic monomers.
Subsequently, several research groups have replaced the stable nitroxyl radical with transition metal species as the capping agents to obtain a variety of copper, nickel, iron, cobalt, or ruthenium-mediated living free radical systems, by so-called atom transfer radical polymerization (ATRP) (see, Matyjaszewski, et al., Macromolecules, 28, 7901, 1995, J. Am. Chem. Soc., 117, 5614, 1995). All of these systems have an apparent central theme, i.e., reversible termination via equilibrium between active and dormant chain end at an elevated temperature, which is regulated by a redox reaction involving metal ions. The main advantage of this reaction is that, through a proper choice of the metal compound, it is possible to operate with a broad spectrum of monomers. However, a major drawback is the formation of a deep colored reaction mixture that requires extensive purification procedures to obtain a desired final product free of coloration.
Other groups have been focusing on another type of living radical initiators, which can initiate living radical polymerization at ambient temperature and form white polymer products. The chemistry was based on the mono-oxidation adducts of trialkylborane as the living radical initiator. The research objective was centered around the functionalization of polyolefins by first incorporating borane groups into a polymer chain, which was then selectively oxidized by oxygen to form the mono-oxidized borane moieties that initiate free radical graft-form polymerization at ambient temperature to produce polyolefin graft and block copolymers (Chung, et al., U.S. Pat. Nos. 5,286,800 and 5,401,805, Macromolecules, 26, 3467, 1993, Macromolecules, 31, 5943, 1998, J. Am. Chem. Soc., 121, 6763 (1999)). Several years ago, several relatively stable radical initiators were discovered, which exhibited living radical polymerization characteristics, with a linear relationship between polymer molecular weight and monomer conversion and produced block copolymers by sequential monomer addition (see Chung, et al., U.S. Pat. Nos. 6,420,502 and 6,515,088, J. Am. Chem. Soc., 118, 705, 1996).
A relatively new method was reported for preparing monotelechelic polypropylene by chemical modification of chain end unsaturated polypropylene (PP) that can be prepared by metallocene polymerization or thermal degradation of high molecular weight PP. (see Chung et al., Macromolecules, 32, 2525, 1999; Polymer, 38, 1495, 1997; Mulhaupt et al., Polymers for Advanced Technologies, 4, 439, 1993; and Shiono et al., Macromolecules, 25, 3356, 1992; Macromolecules, 26, 2085, 1993; Macromolecules, 30, 5997, 1997). Recently, Chung et al. have also reported a facile route to prepare monotelechelic polyolefins containing a reactive functional group (OH, COOH, NH2, etc.). The chemistry is centered on an in situ chain transfer reaction during metallocene-mediated α-olefin polymerization using two reactive chain transfer (CT) agents, including dialkylborane (R2B—H) and styrenic molecule/H2, to form polyolefin containing a reactive alkylborane and styrenic terminal group, respectively. (see Chung et al., U.S. Pat. Nos. 6,248,837 and 6,479,600, J. Am. Chem. Soc. 121, 6763, 1999, Macromolecules 32, 8689, 1999, Macromolecules 34, 8040, 2001, J. Am. Chem. Soc. 123, 4871, 2001, and Macromolecules 35, 1622, 2002). With an appropriate choice of metallocene catalyst, the monotelechelic polyolefin formed shows narrow molecular weight distribution (MW/Mn of 2) and the polymer molecular weight was inversely proportional to the molar ratio of (CT agent)/(α-olefin).
In general, the synthesis of telechelic polymers with a reactive functional group at either chain ends, i.e. “ditelechelic” polymers with functionality of 2, are much more demanding, especially in preparing vinyl polymers. Most of the commercial ditelechelic polymers, such as aliphatic polyesters with two opposing terminal acid or alcohol groups and polyethylene oxide and polypropylene oxide with two opposing terminal alcohol groups, are prepared by polycondensation reaction and ring opening polymerization with suitable initiators, respectively. The preparation of ditelechelic vinyl polymers usually requires the combination of living polymerization and difunctional initiators or functionally substituted initiators. This methodology has been applied to practically all vinyl polymerization techniques, including anionic, cationic, free radical, metathesis, and Ziegler-Natta, which evidence living polymerizations with stable propagating active sites that can be converted to the desired functional group at the chain end. (For examples of anionic living polymerization, see, e.g., U.S. Pat. No. 3,265,765 and D. E. Bergbreiter et al., J. Am. Chem. Soc., 109, 174, 1987; for cationic living polymerization, see, e.g., U.S. Pat. No. 4,946,899; for free radical living polymerization, see Georges, et al., U.S. Pat. Nos. 5,322,912 and 5,401,804, Matyjaszewski, et al., Macromolecules, 28, 7901, 1995 and J. Am. Chem. Soc., 117, 5614, 1995; for metathesis living polymerization, see, e.g., R. H. Grubbs, et al., Macromolecules, 22, 1558, 1989; and for transition metal living polymerization, see, e.g., Y. Doi, et al., Makromol. Chem., 188, 1273, 1987, and H. Yasuda, et al., Macromolecules, 25, 5115, 1992.).
There are also reports describing the preparation of several ditelechelic vinyl polymers by degradation of the polymer that contains some unsaturation units in the polymer backbone. However, it is believed that no one has reported the preparation of ditelechelic polymers containing reactive functional groups at the same polymer chain end. Hence, there is a continuing need for convenient methods for the synthesis of telechelic polymers having varied chemistry.